Field of the Invention
The invention relates to an impregnation process, in which a carrier material is impregnated with an impregnation medium. The invention further relates to a device for monitoring the impregnation of a carrier material with an impregnation medium.
The article titled xe2x80x9cAnalysis of Mold Filling in RTM Processxe2x80x9d by Zhong Cai in the Journal of Composite Materials, Vol. 26, 9/1992 discloses an impregnation process. A resin transfer molding (RTM) process is described, in which fibers are impregnated by a resin to form a composite material. A numerical model is presented, which is based on Darcy""s law and describes the impregnation process.
It is accordingly an object of the invention to provide an impregnation process and a device for monitoring the impregnation of a carrier material, which overcome the above-mentioned disadvantages of the prior art devices and methods of this general type.
With the foregoing and other objects in view there is provided, in accordance with the invention, an impregnation process, which comprises:
impregnating a carrier material with an impregnation medium, so that a dielectric constant of the carrier material changes;
measuring a measured variable dependent upon the dielectric constant; and
determining a functional interrelationship between a reference impregnation and the measured variable and drawing a conclusion from the functional interrelationship as to an impregnation of the carrier material, the functional interrelationship being determined with reference to the steps of:
defining a degree of the impregnation as a ratio of an impregnated to-an unimpregnated volume of the carrier material;
setting up a sequence of degrees of the impregnation with predetermined process parameters, and allocating the reference impregnation with a spatial distribution in the carrier material of at least one unimpregnated dry region and at least one impregnated moist region to each of the degrees of the impregnation by using a simulation with reference to a flow model;
allocating a first dielectric constant to the at least one unimpregnated dry region and allocating a second dielectric constant to the at least one impregnated moist region; and
allocating the measured variable to the reference impregnation with the aid of the first dielectric constant and the second dielectric constant, and thus obtaining the functional interrelationship.
The object directed at specifying an impregnation process is achieved, according to the invention, by an impregnation process in which the carrier material is impregnated with the impregnation medium, so that the dielectric constant of the carrier material changes. The measured variable dependent upon the dielectric constant is measured and the impregnation of the carrier material being determined therefrom.
The invention is based on the finding that the dielectric constant of the carrier material is changed by the penetration of the impregnation medium into the carrier material. Via the determination of the dielectric constant it is possible, for example, to conclude how far the carrier material has been impregnated by the impregnation medium. Thus, it is possible to monitor the impregnation process.
The measured variable that is preferably used is the electrical capacitance of a configuration of electrical conductors, in which configuration the carrier material serves as the dielectric. The configuration in the form of a capacitor enables a simple determination of the dielectric constant via the measurement of an electrical capacitance. The capacitance can, for example, be determined by application of a direct voltage, or alternatively via the determination of a capacitive resistance.
Preferably, a temporal progression of the measured variable is measured and a temporal progression of the impregnation is determined therefrom. On this bases, it is possible to represent the temporal progression of variables characterizing the impregnation process.
Further preferably, a functional interrelationship between a reference impregnation and the measured variable is determined, and a conclusion is drawn from the functional interrelationship concerning the impregnation. The determination of a reference impregnation as a function of the measured variable represents one possibility for obtaining, from the measured variable, a value for the impregnation.
Preferably, the temporal progression of the impregnation is computed in accordance with the interrelationship D(t)=DR(C(t)) where D is the impregnation, DR a reference impregnation, C the measured variable, in particular the capacitance, and t the time. Using the resulting functional interrelationship of the reference impregnation with the measured variable, the temporal progression of the impregnation may be obtained directly through the measured temporal progression of the measured variable with the aid of the above-mentioned interrelationship.
Preferably, the functional interrelationship is determined in that a degree of impregnation is defined as the ratio of the impregnated to the unimpregnated volume of the carrier material. For a sequence of degrees of impregnation with predetermined process parameters, the reference impregnation with a spatial distribution in the carrier material of at least one unimpregnated dry region and at least one impregnated moist region is allocated to each degree of impregnation by use of a simulation with reference to a flow model. A first dielectric constant is allocated to the at least one dry region and a second dielectric constant is allocated to the at least one moist region. A measured variable is allocated to each reference impregnation with the aid of these dielectric constants, and thus the functional interrelationship which is sought is obtained.
The impregnation medium penetrates into the carrier material with the formation of a flow front. In this case, dry regions and/or moist regions may cohere in such a way that they represent in each case one cohesive volume. However, the flow front can also lead to a complex spatial distribution of dry regions and moist regions. Through differing dielectric constants of these regions, results in a complex electric field. By use of the mentioned mode of procedure, it is now possible, even in the case of flow fronts of the impregnation medium in the carrier material which are of more complicated form, to produce a functional interrelationship between an impregnation and the capacitance. This takes place by use of a simulation which is based on a flow model and which delivers, for a predetermined percentage impregnation, a specified distribution of dry regions and moist regions. In that in each instance a specified dielectric constant is allocated to these different regions, it is possible to compute a potential distribution in the carrier material, in dependence upon the spatial distribution of these different regions which becomes apparent. The capacitance is then obtained from the formula   C  =            1      U        ⁢                  ∫        Γ            ⁢                        ϵ          0                ⁢                  ϵ          r                ⁢                              ∇            ϕ                    ·          n                ⁢                  xe2x80x83                ⁢                  ⅆ          s                    
In this case, the integration is carried out over an area xcex93 of electrodes of the conductor configuration which are employed for the capacitance measurement. ∈0 is the dielectric constant of a vacuum, ∈r a mean dielectric constant that is obtained from the dielectric constants of the dry regions and the moist regions, xcfx86 is a potential and U a voltage applied to the electrodes.
Further preferably, for a porous carrier material the spatial distribution of the reference impregnation is computed using Darcy""s law for a flow of a Newtonian impregnation medium or using a suitable modification of Darcy""s law for a flow of a non-Newtonian impregnation medium. For porous media, the computation of the propagation of flow fronts may be simulated with the aid of Darcy""s law, which, as is acknowledged, sufficiently well reproduces the physical conditions.
Preferably, a penetration depth of the impregnation medium into the carrier material is obtained from the impregnation. Further preferably, a temporal progression of the penetration depth is obtained and at least one of the following variables of the carrier material is determined therefrom:
a filtration coefficient,
a flow resistance,
a permeability, and
a relative porosity.
In the case where the impregnation medium flows with a straight flow front through a mold of constant diameter, the following interrelationships apply:                                           q            0                    ⁢                      xe2x80x83                    ⁢                      (            t            )                          =                              ⅆ                          ⅆ              t                                ⁢                      xe2x80x83                    ⁢          E          ⁢                      xe2x80x83                    ⁢                      (            t            )                                              (        1        )                                          K          ⁡                      (                          E              ⁡                              (                t                )                                      )                          =                                                            q                0                            ⁡                              (                t                )                                      ⁢            μ                                              Φ              2                        ⁢                          ⅆ                              ⅆ                t                                      ⁢                          (                                                Δ                  ⁢                                      xe2x80x83                                    ⁢                  p                  ⁢                                      xe2x80x83                                    ⁢                                      (                    t                    )                                                                                        q                    0                                    ⁡                                      (                    t                    )                                                              )                                                          (        2        )                                          μ          ⁡                      (                          E              ⁡                              (                t                )                                      )                          =                                            ⅆ                              ⅆ                t                                      ⁢                          (                                                Δ                  ⁢                                      xe2x80x83                                    ⁢                  p                  ⁢                                      xe2x80x83                                    ⁢                                      (                    t                    )                                                                                        q                    0                                    ⁡                                      (                    t                    )                                                              )                        ⁢            K            ⁢                          xe2x80x83                        ⁢                          Φ              2                                            q            0                                              (        3        )                                          Φ          ⁡                      (                          E              ⁡                              (                t                )                                      )                          =                                                                              q                  0                                ⁡                                  (                  t                  )                                            ⁢              μ                                      K              ⁢                              ⅆ                                  ⅆ                  t                                            ⁢                              (                                                      Δ                    ⁢                                          xe2x80x83                                        ⁢                    p                    ⁢                                          xe2x80x83                                        ⁢                                          (                      t                      )                                                                                                  q                      0                                        ⁡                                          (                      t                      )                                                                      )                                                                        (        4        )                                                                    K              ⁢                              xe2x80x83                            ⁢                              Φ                2                                      μ                    ⁢                      xe2x80x83                    ⁢                      (                          E              ⁡                              (                t                )                                      )                          =                              q            0                                              ⅆ                              ⅆ                t                                      ⁢                          (                                                Δ                  ⁢                                      xe2x80x83                                    ⁢                  p                  ⁢                                      xe2x80x83                                    ⁢                                      (                    t                    )                                                                                        q                    0                                    ⁡                                      (                    t                    )                                                              )                                                          (        5        )            
The symbols that are used are explained in the table below:
For simple geometries of the carrier material, e.g. a cylinder, a simple and uniform flow front results. In this case, an impregnation and thus a penetration depth of the impregnation medium with time may be derived from the determination of the measured variable, without computing a reference impregnation. Via the determination of such a temporal progression of the penetration depth, it is then again possible to obtain a material property such as the filtration coefficient of the flow resistance. Further preferably, a temporal progression of the penetration depth is obtained, and a viscosity of the impregnation medium is obtained therefrom.
Preferably, a resin, in particular an epoxy resin, is used as the impregnation medium. Impregnations with resins and in particular with epoxy resins play a major part, for example, in the manufacture of composite materials and in the impregnation of electric windings of generators. Preferably, the fibers of a fiber composite material to be manufactured are used as the carrier material.
Preferably, an electrical insulating material is used as the carrier material. Further preferably, the electrical winding of a stator of a generator, in particular of a turbogenerator, is impregnated, in particular in a total impregnation process (VPI process, VPI=Vacuum Pressure Impregnation). Nowadays, turbogenerators are in many cases manufactured via a total impregnation of their stator. In this case, the stator is flooded with epoxy resin and impregnated by the epoxy resin under pressure and at high temperature. Following completion of curing of the epoxy resin, the result is accordingly a particularly durable and resistant stator cladding.
Preferably, from the measured impregnation a conclusion is drawn as to the presence of an impregnation defect in the carrier material. Impregnation defects are permanently unimpregnated regions. The measurement of the impregnation with the aid of the measured variable makes it possible, for example, to detect impregnation defects occurring by reason of material properties. An indication of an impregnation defect may in particular be that the customary final capacitance, i.e. the capacitance that customarily becomes apparent at the end of the impregnation process, is not attained.
According to the invention, the object directed at specifying a device is achieved by a device for monitoring the impregnation of the carrier material with the impregnation medium, in which a conductor configuration is disposed in such a way that the carrier material serves as a dielectric influencing the capacitance of the conductor configuration and in which a measuring device for measuring the electrical capacitance is connected to the conductor configuration.
The advantages of such a device are obtained in a manner corresponding to the above statements concerning the advantages of the impregnation process.
Preferably, the carrier material is an insulation of a conductor rod of the stator of the turbogenerator. In this case, the conductor configuration is an electrically conductive strip that surrounds the insulation and embraces the electrical conductor of the conductor rod. The impregnation during a total impregnation process of the stator of the turbogenerator may be monitored in a particularly simple manner by this configuration. It is merely necessary to measure the capacitance between the electrically conductive strip and the electrical conductor of the conductor rod. Via the capacitance measurement, it is possible to draw a conclusion as to the impregnation in accordance with the above statements concerning the impregnation process.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in an impregnation process and a device for monitoring the impregnation of a carrier material, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.